A conventional and general induction heating apparatus, such as an induction heating cooker, will be described referring to FIG. 20 to FIG. 22.
FIG. 20 is a cross-sectional view of a conventional induction heating cooker. In the figure, a pan or an object 1 to be heated having the shape of the pan is placed on a plate 4 provided above an induction heating section 3 having a heating coil 2. The plate 4 is, for example, an insulation plate made of a ceramic material or the like having a thickness of 4 mm and covers the upper portion of a housing 10. The heating coil 2 to which a high-frequency current is supplied from a drive circuit 5, generates a high-frequency magnetic field and provides the high-frequency magnetic field to the object 1 to be heated. Under the heating coil 2, a plurality of magnetic members 6, made of a magnetic material having high magnetic permeability, such as ferrite, are provided so that the high-frequency magnetic field generated from the heating coil 2 is directed toward the object 1 to be heated, thereby improving efficiency. On the bottom face of the plate 4, a conductive coating film 7 made of carbon or the like is formed by printing or the like. The conductive coating film 7 is connected to the earth or the input section or the output section of a rectifier (not shown) via a capacitor 8.
When a high-frequency current flows in the heating coil 2, a high-frequency magnetic field is generated around the heating coil 2, and an eddy current owing to electromagnetic induction is generated at the bottom portion of the object 1 to be heated, thereby heating the object 1 to be heated. A leak current leaking from the heating coil 2 to the earth owing to the high-frequency high voltage and stray capacitance generated in the heating coil 2 is restricted by the electrostatic shielding action of the conductive coating film 7.
In the above-mentioned conventional induction heating cooker, by the interaction between the current induced in the bottom portion of the object 1 to be heated and the current of the heating coil 2, a force of repulsion away from the heating coil 2 is generated at the bottom portion of the object 1 to be heated. In the case when the object 1 to be heated is made of a material having high magnetic permeability and relatively large resistivity, such as iron, the current value required for obtaining a desired heating output may be small, whereby this repulsion force is relatively small. In addition, in the case of iron or the like, a magnetic attraction force is exerted by the magnetic flux flowing in the object 1 to be heated, whereby there is no fear of floating or dislocating the object 1 to be heated.
In the case when the object 1 to be heated is made of a material having low magnetic permeability and high electrical conductivity, such as aluminum or copper, in order that a desired heating output is obtained, it is necessary to increase the current flowing in the heating coil 2 and to induce a large current in the object 1 to be heated. As a result, the repulsion force is increased. In addition, since such a magnetic attraction force as generated in the case of a material having high magnetic permeability, such as iron, is not exerted on the object 1 to be heated which is made of aluminum, a large force is exerted in the direction of moving the object 1 to be heated away from the heating coil 2 by the action of the magnetic field of the heating coil 2 and the magnetic field of the induction current. This force is exerted on the object 1 to be heated as buoyancy. In the case when the weight of the object 1 to be heated is light, the object 1 to be heated has a fear of being floated and moved over the placement face of the plate 4 by this buoyancy.
FIG. 21 (a) is a view seen from the side of the object 1 to be heated, showing the direction of the current flowing in the heating coil 2, and FIG. 21 (b) is a view seen from the same direction as that of FIG. 21 (a), showing the direction of the eddy current flowing in the object 1 to be heated owing to the induction of the current flowing in the heating coil 2. As shown in FIGS. 21 (a) and 21 (b), the eddy current flowing in the object 1 to be heated has a direction opposite to that of the current flowing in the heating coil 2 and has nearly the same loop shape as that of the current. These two circular currents generate the same phenomenon as that generated when the poles, having the same polarity (for example, the N pole to the N pole), of two permanent magnets having substantially the same cross-sectional area as the area of the heating coil 2 are opposed to each other. In other words, a large repulsion force is generated between the object 1 to be heated and the heating coil 2.
This phenomenon is remarkable in the case when the material of the object 1 to be heated is a substance having low magnetic permeability and high electrical conductivity, such as aluminum or copper. Although nonmagnetic stainless steel is a material having low magnetic permeability likewise, it is a material having an electrical conductivity lower than those of aluminum and copper, whereby sufficient heat generation is obtained even if the current flowing in the heating coil 21 is small. Hence, the eddy current flowing in the object 1 to be heated is small, and the repulsion force generated in the object 1 to be heated is also small.
FIG. 22 is a graph showing an example of the relationship between input power and buoyancy at the time when the object 1 to be heated which is made of aluminum is heated. In the graph of FIG. 22, the horizontal axis represents the input power (W: watt) and the vertical axis represents the buoyancy (g: gram). As known from this figure, the buoyancy increases as the input power increases; therefore, when the buoyancy exceeds the weight of the object 1 to be heated, the object 1 to be heated is dislocated or floated.
The following are examples of the prior arts regarding the floating of the object 1 to be heated in an induction heating cooker. In Japanese Laid-open Patent Application Sho 61-128492 and Japanese Laid-open Patent Application Sho 62-276787, the floating and movement of the object to be heated are detected by using a weight sensor. In Japanese Laid-open Patent Application Sho 61-71582, the position of the object to be heated is detected by using a magnetic sensor. In Japanese Laid-open Patent Application Hei 4-765633, the fact that the object to be heated is moved by buoyancy is detected by using resonance frequency detecting means.
In either of the above-mentioned prior arts, in the case when a buoyancy of a predetermined value or more is exerted on the object to be heated or when the fact that the object to be heated is floated or moved is detected, the electric power for heating is restricted or the heating itself is stopped so that further floating or movement does not occur. However, since the electric power for heating is restricted, sufficient heating power is not obtained, thereby causing a problem of getting into a situation wherein cooking is interrupted in some cases.
For example, in the case of heating an object to be heated which is a Yukihira pan made of aluminum having a weight of 300 g and supplied with water of 200 cc, 500 g in total weight, the buoyancy becomes larger than the total weight of the pan and the substance (water) to be cooked of 500 g, at an input power of about 850 W as shown in FIG. 22. Hence, the pan is floated, and it is difficult to heat at higher input power. In the case when an aluminum pan for example is detected in the above-mentioned prior arts, the input power is restricted to a value not more than the input power at which the pan is floated, 800 W for example, thereby preventing the pan from being floated. However, according to experiments conducted by the inventors, even if heating was carried out at an input power of 800 W, it was difficult to heat the above-mentioned water of 200 cc to a boiling state. Therefore, the cooker is very low in heating performance as an induction heating cooker capable of heating a pan made of aluminum.